The present invention relates generally to the field of semiconductor processing systems such as ion implanters, and more specifically to an ion source for use in such an ion implanter, having a replaceable and sputterable solid source material contained therein.
In the large-scale manufacture of integrated circuits, ion implantation has become a standard accepted technology of industry to dope workpieces such as silicon wafers with impurities. Conventional ion implantation systems include an ion source that ionizes a desired dopant element which is then accelerated to form an ion beam of a prescribed energy. The ion beam is directed at the surface of the workpiece to implant the workpiece with the dopant element. The energetic ions of the ion beam penetrate the surface of the workpiece so that they are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity. The implantation process is typically performed in a high vacuum process chamber which prevents dispersion of the ion beam by collisions with residual gas molecules and which minimizes the risk of contamination of the workpiece by airborne particulates.
Ion dose and energy are the two most important variables used to define an implant step. Ion dose relates to the concentration of implanted ions for a given semiconductor material. Typically, high current implanters (generally capable of generating tens of milliamps (mA) of ion beam current are used for high dose implants, while medium current implanters (generally capable up to about 1 mA beam current) are used for lower dose applications. Ion energy is used to control junction depth in semiconductor devices. The energy levels of the ions that comprise the ion beam determine the degree of depth of the implanted ions. High energy processes such as those used to form retrograde wells in most semiconductor devices require implants of up to a few million electron volts (MeV), while shallow junctions may only demand energies below 1 thousand electron volts (keV).
Conventional ion sources utilize an ionizable dopant gas that is obtained either directly from a source of a compressed gas or indirectly from a solid from which has been vaporized. Typical source elements are boron (B), phosphorous (P), gallium (Ga), indium (In), antimony (Sb), and arsenic (As). These source elements can be provided in solid form or in gaseous form, such as boron, which may be provided in either solid form (B) or in gaseous form as boron trifluoride (BF3). BF3, however, suffers from the disadvantages of being toxic and flammable, as well as being corrosive to implanter surfaces due to its fluorine component.
A typical ion source 10 for obtaining atoms for ionization from a solid or gaseous form is shown in FIG. 1. This type of ion source comprises an arc chamber AC, which functions as an anode, and a filament F contained therein which functions as a cathode. In operation, an arc voltage is applied between the filament F and the walls of the arc chamber AC. The energized filament thermionically emits high-energy electrons E that are accelerated toward the electrically grounded (i.e., relatively positively biased) chamber wall. A gas containing boron or phosphorous is fed into the arc chamber AC via an inlet I.
A repeller R is positioned within the arc chamber AC opposite the filament F. The repeller electrostatically repels the filament-emitted electrons E to confine these electrons to a path P1 within an ionization region between the filament and the repeller. The electrons E collide with and dissociate and/or ionize the gas molecules in the ionization region, where the number of collisions with ionizable gas molecules is maximized. Positive ions are created when an electron is removed from the outer shell of these gas molecules by the filament-emitted electrons E. In this manner, a plasma is created comprised at least partially of positively charged ions. A generally positively charged ion beam is drawn from this plasma, typically through a source aperture SA in the arc chamber, by means of an electrode at negative bias (not shown).
A source magnet SM increases ionization efficiency in the arc chamber by setting up a magnetic field along the chamber. The magnetic field causes the path P1 of the electrons E traveling through the arc chamber to be helical, which further increases the yield of collisions with the gas molecules, thereby creating more useful ions. The source magnet current is adjusted to maximize the extracted ion beam current and ion beam quality.
The repeller R is typically made of metal, for example, molybdenum (Mo). The repeller is permitted to reside at a floating electrical potential. A ceramic insulator C insulates both the filament and the repeller from the walls of the arc chamber, which are typically maintained at ground potential. The filament F and the repeller R are thereby electrically and thermally isolated from each other and from the arc chamber walls.
When ions need to be obtained from a solid source, the walls of the arc chamber may be constructed of or lined with a sputterable source material such as boron, as in Japanese Patent Application No. 96JP-356494, filed Dec. 26, 1996 (Publication No. 10-188833, published Jul. 21, 1998). In such a sputter ion source, an inert carrier gas such as argon (Ar) is fed into the arc chamber AC via inlet I and is ionized by the filament F to create an ionized plasma. The ionized plasma then sputter etches material from the boron liner, which in turn is dissociated and/or ionized by the electrons emitted from filament F. The resulting positive boron ions and positive argon ions are extracted through the source aperture SA in the form of an ion beam. The ion beam is subsequently mass analyzed to remove the argon ions to produce an ion beam comprised substantially of ionized boron atoms.
Known sputter ion sources, however, require the sputterable wall liners to be replaced after they have been sufficiently eroded by the sputtering process. In addition, the repeller must be maintained, as it may become eroded over time. Still further, if the sputterable wall liners are changed to effect a different ion species (i.e., from boron (B) to phosphorous (P)), previously sputtered material coated on the repeller may pose a risk of ion species contamination. Thus, to effect an ion beam species change, both the sputterable wall liners and the repeller must be changed.
Accordingly, it is an object of the present invention to provide a mechanism for including a sputterable solid source material for an ion implanter ion source, while minimizing the maintenance required to effect a change of source materials. It is a further object of the invention to provide a repeller for an ion implanter ion source that functions both as a repeller and as a sputterable solid source material. It is yet a further object of the invention to provide a mechanism for controlling the characteristics of the ion beam by actively controlling the voltage applied to the repeller.
An ion source for an ion implanter is provided, comprising: (i) an ionization chamber defined by chamber walls, and having an inlet into which a sputtering gas may be injected and an aperture through which an ion beam may be extracted; (ii) an ionizing electron source for ionizing the sputtering gas to form a sputtering plasma; and (iii) a sputterable repeller disposed within the chamber. The sputterable repeller both (i) repels electrons emitted by the electron source, and (ii) provides a source of sputtered material that can be ionized by the electron source. The sputterable repeller comprises a slug of sputterable material, and further comprises mounting structure for mounting the slug within the ionization chamber, so that the slug is made removably detachable from the mounting structure.
The sputterable material may be any of the following elements, or a compound including any of these elements: aluminum (Al), boron (B), beryllium (Be), carbon (C), cesium (Cs), germanium, (Ge), molybdenum, (Mo), antimony (Sb), or silicon (Si). The repeller is negatively biased with respect to the ionization chamber walls, and may be continuously variably biased to provide for a wide dynamic range of resulting ion beam currents.